Alienware 13 r3 performance problems

[miniCotulla]

Hi, I got myself a used Alienware 13 r3 (GTX1060, Core I7 6700HQ, 16gb RAM, clean Windows 10 install), everytime I start a game it runs fine for the first 15min, it gets very hot but doesn't thermal throttle. I also did the PCH heatsink-mod to keep it from reaching 90C° and bottlenecking performance. It's a really strange behavior, the gpu stays under 90C° and boosting fine, the CPU fluctuates between base clock (2.6ghz) and 3ghz boost, Intel XTU states it's power limit throtteling while staying at 35 watts the whole time, shouldn't 35 watts be fine for this cpu? Also I did a repaste. But still after 15mins the framerate drops(60ish FPS to 10) and after a few seconds it goes up again. I undervolted the cpu (-140mV) and that seems to fix the problem(CPU now stays at 3ghz boost constantly) but I can't get the undervolt to stick between reboots and waking the notebook up from sleep. Also I've read that I should undervolt the cache and maybe the iGPU, does this really help if I'm using the GTX1060? Maybe undervolting the GTX1060 would also help? What should I do to fix performance and make it stick so I don't have to dial it in everytime I reboot?